The SNB had defended a minimum of 1.20 francs per euro since September 2011, buying euros in size to hold it while the franc's safe-haven-currency status pulled the other way. The commitment had been restated as recently as days before it was abandoned.
When the floor was withdrawn without warning, EUR/CHF fell through 1.20 and traded far below parity within minutes before settling nearer 1.04 that day. There was effectively no liquidity on the way down, so stops did not fill near their levels, they filled wherever the next price appeared, an extreme case of slippage-tolerance being irrelevant.
The consequences reshaped the retail industry. Clients were left with balances far beyond their deposits, which is why negative-balance-protection moved from a marketing feature to a regulatory requirement in several jurisdictions. At least one UK broker entered insolvency and another required a large emergency loan, illustrating counterparty-risk in practice.
Example: a client short EUR/CHF was fine; a client long 5 lots from 1.2010 with a stop at 1.1950 was filled far lower. At a 0.9000 fill that is 30,100 pips against, roughly CHF 150,500 on an account that may have held a few thousand.
Related: negative-balance-protection, currency-peg, swissy, counterparty-risk